I was kind of hoping that you had a game, set and match answer that I could use when discussing this with other Catholics.
Is there any canonical requirement for there to be intent? I thought that validity was simply a question of matter, form and minister.
Yes, there has to be intent, but the argument is over what that means. There clearly has to be some intent because it has to constitue a human act. There's actually a papal condemnation somewhere of the proposition that an "internal intention" is not required for the validity of the Sacrament. But my argument is that this does not mean what a lot of people are trying to turn it into, which is that you have to internally intend what the Church intends. No, you have to internally intend to DO what the Church does. Basically, this is mean to exclude situations where it is evident in the external forum that you don't intend to do what the Church does, so, for instance, if some kids are playing and in the context of that play go administer Baptism to someone (with the current matter and form), or if someone were to do that as part of a movie performance or a play. They're not intending to do what the Church DOES but merely to imitate the ceremony.
Similarly, if you had a priest who was on some pain meds and got up in a mindless/incoherent state said a Mass, it would not be valid, since he didn't intend to do anything and wasn't even performing a human act.
But if I'm a priest and I go into the sacristy and put on my vestments, and then go out there at the time schedule for Mass in the bulletin, and peform the Rite of the Church ... which I intend to DO "internally" ... I could think in my head all I want that "I don't intend to consecrate. I don't intend to transubstantiate." and it matters nothing. You still intended to DO what the Church does, i.e. to PERFORM the Rite that the Church prescibes, wherein the CHURCH intends to consecrate. Valid Mass regardless of any such schizophrenic mindgames. It's very similar to the loaded gun above. If you pull the trigger on the loaded gun, you absolutely intended to DO what it takes to KILL the man, even if in your head you keep reiterating the mantra "I do not wish to kill him. I do not wish to kill him." No, you intended to DO that which WOULD almost inevitably kill him, so you intended to kill him. Of course, even if, say, the gun jams, you're still formally guilty of attempted murder ... but that's a separate issue.